Lie about origin of synthetic threads (6)
I believe the answer is:
fibres
'threads' is the definition.
(fibre is a type of thread)
'lie about origin of synthetic' is the wordplay.
'lie' becomes 'fib' (fib is a kind of lie).
'about' says the letters should be written backwards.
'origin of synthetic' becomes 'ser' (I can't explain this - if you can you should believe this answer much more).
'ser' reversed gives 'res'.
'fib'+'res'='FIBRES'
(Other definitions for fibres that I've seen before include "Threadlike substances" , "Long thin substances, threads" , "Threads or tendrils" , "Filaments" , "They may be spun" .)